Microwave and millimeter-wave technologies have been widely used for position sensing, such as described in Stezer et al., “Microwave position sensor with sub millimeter accuracy,” IEEE Trans. Microwave Theory and Techniques, vol. 47, pp. 2621-2624, December 1999. Microwave and millimeter techniques have also been used for precision noise measurement, such as described in Ivanov et al., “Microwave interferometry: Application to precision measurements and noise reduction techniques,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 45, pp. 1526-1536, November 1998. Likewise, microwave and millimeter-wave methods have been applied to displacement measurement, such as described in Kim et al., “On the development of a multifunction millimeter-wave sensor for displacement sensing and low-velocity measurement,” IEEE Trans. Microwave Theory and Techniques, vol. 52, pp 2503-2512, November 2004.
In addition, microwave and millimeter technologies have been applied to cardio pulmonary sensing, such as described in Droitcour et al., “Range correlation and I/Q performance benefits in single-chip silicon Doppler radars for noncontact cardiopulmonary monitoring,” IEEE Trans. Microwave Theory and Techniques, vol. 52, pp. 838-848, March 2004. The mechanism of most of the microwave displacement-related measurement systems is the detection of the phase shift caused by the movement of the target. Based on this, a Doppler radar has been developed to monitor periodic vital sign movements, and a linear approximation was used to analyze the performance as shown by Droitcour et al. However, the system could only detect the frequency of movement, not the amplitude.
A microwave Doppler radar sensor has been reported to be able to measure both displacement and velocity of one direction movements by measuring Doppler frequency shift. S. Kim, and Cam Nguyen, “On the development of a multifunction millimeter-wave sensor for displacement sensing and low-velocity measurement,” IEEE Trans. Microwave Theory and Tech. vol. 52, pp. 2503-2512, November 2004. (Kim et al.) The recently discovered nonlinear Doppler phase modulation effect, which would generate harmonics of the movement frequency when the movement amplitude is comparable to the carrier wavelength, has brought out an alternative method to obtain the pattern of a purely sinusoidal movement. C. Li and J. Lin. “Non-Contact Measurement of Periodic Movement by a 22-40 GHz Radar Sensor Using Nonlinear Phase Modulation,” IEEE MTT-S Int. Microwave Symp. Dig., pp. 579-582, Honolulu, June 2007. (Li et al.) In order to eliminate the residual phase effect that will result in the null detection point problem identified by Yan et al. (Y. Yan, C. Li, and J. Lin, “Ka-band Quadrature Doppler Radar System with Sub-millimeter Resolution and Sensitivity in Measuring Periodic Movement,” IEEE Wireless and Microwave Technology Conference, 12-13, April 2010), Li et al. take the amplitude ratio between two even order or two odd order frequency components on the baseband spectrum to cancel out the associated residual phase, leaving the ratio determined only by the Bessel function of the movement amplitude. Sine the movement frequency can be read directly from the baseband spectrum, after obtaining the value of movement amplitude from the harmonic amplitude ratio, the originally unknown periodic movement can be reconstructed.
The microwave Doppler radar in Kim et al. requires high-sampling rate to track the instantaneous velocity of a moving target, and can, therefore, be limited to measure low-velocity movements. On the other hand, the detection method based on nonlinear Doppler phase modulation effect does not need to measure the instantaneous velocity, and the sampling rate of the radar system is relaxed to only a few times (two or three) of the fundamental frequency of the periodic movement. The residual phase problem can limit the radar in Li et al. to measuring sinusoidal movements. When the movement pattern of the target is a non-sinusoidal periodic waveform, such as multiple sine waves of different frequencies, the detection method of Li el al. may not be effective.
Accordingly, there is a need for a method and apparatus for accurate non-contact measurement of frequency and amplitude of mechanical vibration that can be non-sinusoidal.